Surface plasmon waves are electromagnetic waves which may exist at the boundary between a metal and a dielectric (hereinafter referred to as the "sample"). Such waves can be excited by light which has its electric field polarized parallel to the incident plane (i.e., transverse magnetic (TM) polarized). When the parallel component of the propagation constant of the incident light equals the real part of the surface plasmon wave propagation constant, the incident light resonantly excites the surface plasmon waves, and a fraction of the incident light energy is transferred or dispersed to surface plasmon resonance (SPR). This dispersion of energy depends on both the dielectric constant of the metal and that of the sample in contact with the metal. By monitoring the resonance wavevector of the metal/sample interface, the dielectric constant of the sample (gas or solution) may be obtained. Alternatively, if the sample is contaminated by a chemical species, dielectric constant measurements may provide the concentration of the chemical species in the sample.
Traditionally, SPR, has been measured using the Kretschmann configuration (Kretschmann and Raether, Z. Naturforsch. Teil A 23:2135-2136, 1968). In this configuration, a thin layer of highly reflective metal (such as gold or silver) is deposited on the base of a prism. The metal surface is then contacted with the sample, and the SPR reflection spectra of the sample is measured by coupling TM polarized, monochromatic light into the prism and measuring the reflected light intensity as a function of the angle of incidence. The angle of minimum reflective intensity is the resonance angle at which maximum coupling occurs between the incident light and the surface plasmon waves. This angle, as well as the half-width of the resonance spectrum and the intensity at the angle of minimum reflective intensity, may be used to characterize or sense the sample which is in contact with the metal surface (Fontana et al., Applied Optics 27:3334-3339, 1988).
Optical sensing systems have now been constructed based on the Kretschmann configuration described above. Such systems utilize the sensitivity of SPR to changes in the refractive indices of both bulk and thin film samples, as well as to changes in the thickness of thin films. These systems, in conjunction with appropriate chemical sensing layers, have led to the development of a variety of SPR-based chemical sensors, including immunoassay sensors (e.g., Liedberg et at., Sensors and Actuators 4:299-304, 1983; Daniels et al., Sensors and Actuators 15:11-17, 1988; Jorgenson et al., IEEE/Engineering Medicine and Biology Society Proceedings 12:440-442, 1990), gas sensors (e.g., Liedberg et al., supra; Gent et al., Applied Optics 29:2843-2849, 1990), and liquid sensors (e.g., Matsubaru et at., Applied Optics 27:1160-1163, 1988).
While the Kretschmann configuration for SPR-based chemical sensors offers significant sensitivity, their relatively large size has severely restricted their application. For example, these bulk optic sensing systems are limited by their use of a coupling prism causing such systems to be relatively large, expensive, and inapplicable for remote sensing applications. Moreover, such sensors generally require a monochromatic light source, are expensive to manufacture due to configuration constraints (such as the presence of a prism), and require that the incident light sweep over a broad range of incidence angles.
Accordingly, there is a need in the art for an improved SPR sensor, as well as for apparatus and methods relating thereto. Specifically, there is the need for an SPR sensor which readily permits remote sensing, is inexpensive, and is free from the limiting constraints now present with existing SPR-based chemical sensors.